This invention relates to improvements in cyclone contactors of the type used in sample evaporators and emission monitoring systems and, in particular, to a means for maintaining the proper liquid level in the contactor receptacle without the need for an external sensor.
Cyclone contactors are employed as concentrators in emission monitoring systems as shown and described, for example, in U.S. Pat. Nos. 4,117,714 and 5,011,517. They are also utilized as the key component in laboratory analysis systems that rapidly evaporate samples under analysis in order to concentrate the analyte, as shown and described in U.S. Pat. No. 5,679,580. In these and other applications the cyclone concentrator typically comprises a cylindrical receptacle having an elongated inlet slit therein arranged to introduce air or other gas under pressure into the cylindrical chamber in an tangential direction where it swirls in the receptacle in intimate contact with a liquid introduced through a separate inlet port. An external sensor has been employed to maintain the liquid at a level below an instability point relative to the length of the gas inlet slit in order to prevent the liquid from being expelled from the receptacle by the vacuum that draws the air or other gas into the chamber for the swirling action.
Several types of sensors have been utilized to monitor the liquid level inside the contactor and open a valve as necessary to admit fluid. These include sensors of the conductive, capacitive and optical type, each of which has drawbacks that limit its usefulness. For example, a conductive sensor has probes that are inserted through the wall of the contactor receptacle, thus eliminating any variability in the level sensing; a capacitive sensor can be mounted externally in such a way to allow level variability, but it cannot sense collection fluids that are non-conductive; and a through-beam optical sensor is subject to interference by degradation of and deposits on the inner surface of the receptacle.
In addition, the valves controlled by these sensors are digital, i.e., open or closed. The hysteresis necessary in the valve control allows fluid into the chamber in small bursts that take the liquid level higher than the sensing position before the valve is closed. Thus, the liquid level in the contactor must be held lower than the instability point to ensure that no liquid is withdrawn by the negative pressure.